The four-wave mixing in an optical fiber is generated, resulting from the third-order non-linear polarization of a fiber material. The present application discusses in particular the phenomenon in which idler light (converted light) of a frequency fc (=2fp−fs) is generated when probe light (or signal light) of a frequency fs and pumping light of a frequency fp present. Using four-wave mixing would make it possible to generate a frequency converted light of a desired frequency fc by creating pumping light at an appropriate frequency fp on the frequency axis for inputted signal light of a frequency fs. A device that implements generating four-wave mixing with high generation efficiency is used as fiberoptic non-linear device in the field of optical communications. Specific ways of using it include an optical wavelength converter [1–6], an optical parametric amplifier (CPA) [8–11], the compression of pulses [2], the generation of phase conjugated light [7], an optical signal regeneration circuit [12], and a multi-wavelength optical source [13] to be used in wavelength division multiplexing communications. Incidentally, in the specification of the present application, a number in [ ] denotes the number of a literature shown in the list attached to the specification of the present invention.
The above-mentioned phenomenon takes place when pumping light is present at two different frequencies f1 and f2. Hereinafter in this specification, for the sake of convenience, the phenomenon in which the pumping light is present at one frequency is referred to as the three wave mixing (TWM), while the phenomenon in which the pumping light is present at two different frequencies is referred to as the four wave mixing. Both phenomena are discussed in the same frame in which four photons interact with one another from the viewpoint of quantum theory. Therefore, it is to be understood that when a discussion that applies to both occurs, the phenomenon is called four-photon mixing (FPM) collectively.
In general, it is possible to implement the FPM under optimum circumstances by allowing the zero dispersion wavelength of a fiber and the wavelength of pumping light to coincide with each other on the wavelength axis. This is because the phase mismatch of the propagation constant 13 is minimized [8]. In the case where the dispersion of an optical fiber at the wavelength of pumping light lies in anomalous dispersion region and the pumping light power is sufficiently intense, the effects of the self phase modulation (5PM) and the cross phase modulation (XPM) would cause a variation of the phase matching condition. It was also confirmed that this allowed the phase matching to be achieved, depending on the intensity of pumping light in the anomalous dispersion region of an optical fiber.
Devices that employ the FPM in the optical fiber have a variety of applications. For example, such applications may include wavelength conversion in terms of the construction of an optical network in wavelength division multiplexing communications. In this case, it is desired to convert, multi channels signal light in a broad bandwidth with conversion efficiency less dependent on wavelength and with small loss due to the conversion [4]. This is physically equivalent to generate the third-order non-linear effect in an optical fiber in a broader bandwidth with higher efficiency. For the sake of this purpose, the following two methods and their combination are conceivable. (1) Non-linear effect is generated by launching the high-power pump light into the fiber or by using an optical fiber having enhanced non-linearity per unit length. (2) The fiber is elongated to provide light with more time for performing non-linear interaction in the optical fiber in order to generate non-linear effect. Among these two ways of thinking, the idea (1) is very important.
Making the fiber shorter would lead to prevent variations of dispersion in the fiber. In literatures [6, 7], to compensate for the shortage in length of the interaction, three highly nonlinear dispersion shifted optical fibers (HNL-DSF), each 250 m in length and having the zero dispersion wavelength substantially at the same point of frequency, are prepared and connected to each other to realize a long fiber (a length for non-linear interaction of light in the fiber). Enhanced non-linearity of each of the optical fibers can generate effective FPM even in a fiber having a total length of as much as 750 m.
Making the fiber shorter provides other various advantages such as the prevention of the stimulated Brillouin scattering, the prevention of degradation of FPM generating efficiency due to a difference in polarization and reduction in loss, which are currently considered to have practically the advantage over (2) mentioned above. As described in the foregoing, it is possible to implement a fiberoptic device actively employing FPM such as wavelength converter or OPA by shortening an optical fiber or a FPM medium. Nowadays, highly nonlinear optical fibers are eagerly being developed in order to provide the shortened optical fiber with greater non-linearity [15–17].
When the aforementioned highly nonlinear optical fiber is used to produce a wavelength converter, the wavelength converter is to be produced by combining a highly nonlinear optical fiber having a given length and given non-linearity per unit length and the light source of pumping light having the maximum output limited by its specification. The conditions must be conceived for constituting a wavelength converter that satisfies the desired characteristics with the given fiber and light source.
Concerning this problem, for example in literatures [1–3], the inventors have provided a solution of giving the minimum value of the intensity of pumping light to be inputted with the fiber being kept constant in length. This minimum value corresponds to the threshold value of the pumping light required to generate optical parametric amplification in an anomalous dispersion region of the optical fiber. More specifically, this threshold value is determined by the frequency of the signal light and pumping light at the time of generating TWM and the propagation constant of light in the fiber, shown by the following equation using the phase mismatch Δβ of the propagation constant and the non-linear coefficient γ of the fiber.
That is,Pp≧Pth=−Δβ/(4γ)  (1)
Wavelength conversion with high conversion efficiency can be realized by setting the pumping light intensity Pp to be greater than the threshold value. Δβ<0 is achieved only when the dispersion of the fiber at the wavelength of the pumping light is in the anomalous dispersion region and equation (1) is significant (under the conditions). The above-mentioned fact can be in principle derived from an approximate solution [8, 9] of the TWM given by Stolen and Bjorkholm.